The present invention relates to systems and devices for measuring concentrations of nanometer or ultrafine particles, and more particularly to such systems that are adjustable in terms of their sensitivities to certain sizes or electrical mobilities of particles or sets of particles within the nanometer range.
When materials are produced or formed in the nanometer size range, i.e. from about 0.1 micrometers in diameter down to molecular levels, they exhibit unique properties that influence their physical, chemical and biological behavior. Nanotechnology, the field of endeavor concerned with materials in this size range, has experienced explosive growth over the last several years as new and diverse uses for nanomaterials are discovered and developed throughout a broad range of industries.
These developments have raised concerns, because the occupational health risks associated with manufacturing and using nanomaterials are not clearly understood. Many nanomaterials are formed from nanoparticles initially produced as aerosols or colloidal suspensions. Workers may be exposed to these particles through inhalation, dermal contact and ingestion, at increased levels due to working environments with nanoparticles in concentrations that far exceed ambient levels. The present invention is concerned with exposure due to inhalation.
Traditionally, health related concerns about airborne particles have focused on particle concentrations in terms of mass per unit volume. Under this approach, permitted maximum concentration standards are determined, and mass concentrations are measured with respect to these standards. However, toxicologic studies involving ultrafine particles (0.1 micron diameter and below) suggest that particle surface area, as compared to either particle number or particle mass, is the better indicator of health effects. This may follow from the fact that for any given shape (e.g. spherical), the smaller the particle, the greater is its surface area compared to its volume or mass. A proportionally larger specific surface area (i.e. surface area divided by mass) increases the tendency of a particle to react with chemicals in the body. Moreover, due to the small mass of nanoparticles, mass concentration measurements are difficult to obtain and lack the requisite sensitivity, even when based on particle accumulation such as through collection of particles on a filter. Particle measurements based on number concentrations are more sensitive, but subject to increased losses and reduced counting efficiency in the nanometer size range. Accordingly, instruments that measure particle concentrations in terms of surface area, especially accumulated or aggregate surface area, are expected to provide more useful assessments of health risks due to nanoparticle exposure.
Another prominent factor influencing the impact of nanoparticle exposure is the region of the respiratory system in which the inhaled nanoparticles are deposited. Deposition in the head (naso-pharyngeal) region raises a risk of particles reaching the brain. In the TB region, cilia tend to remove deposited particles by pushing them toward and into the esophagus. However, particles deposited in the alveolar region are more likely to be transferred to the blood, and less likely to be expelled, because of a less efficient clearing mechanism. The chart of FIG. 1 shows head (H), TB and A region deposition curves (deposition percent vs. particle diameter) over a range of diameters from 1 to 100 nm. The deposition curves are based on the International Commission on Radiological Protection (ICRP) Dosimetry Model, and more particularly were obtained using a computer program known as “LUDEP” available from the UK National Radiological Protection Board. As seen from the curves, alveolar deposition becomes more prominent as particle diameters increase above about six nanometers.
Another factor influencing nanoparticle deposition, and thus health effects, is the level of physical activity. The chart in FIG. 2 plots percent deposition as a function of particle diameter for a variety of activity levels associated with nasal breathing. The curves show deposition in the A and TB regions, over a particle diameter range of about 3.5 to 410 nm, again according to the ICRP Dosimetry Model. In the alveolar region over the ultrafine particle range, higher levels of activity increase the overall deposition percent, and shift deposition toward an increased proportion of smaller particles. In the tracheobronchial region, higher levels of activity reduce the overall deposition percent, but again shift the deposition toward a higher proportion of smaller particles. Finally, deposition varies with the type of individual, based upon such factors as age, sex, size and physical condition.
In FIG. 3, the head, tracheobronchial and alveolar deposition curves from FIG. 1 are weighted to show deposition in terms of surface area concentration, and further are normalized to a sensitivity of 1.0 at a diameter of 100 nm, to show sensitivity as a function of particle diameter. This provides response functions that respectively indicate head, tracheobronchial and alveolar deposition in terms of particle surface area. The chart also shows the geometric surface area (Dp2) function and a number concentration (Dp0) function, both of which appear as straight lines on the log/log scale.
Over most of the 10-100 nm size range, the H, TB and A region response functions are generally linear and have slopes more gradual than that of the Dp2 function. These functions become less linear and diverge toward the Dp2 function as particle diameters decrease. Instruments that employ diffusion charging of aerosol particles, followed by collection of the charged particles to measure the resultant electrical current, tend to correspond more closely to particle diameters than particle surface areas in the particle diameter range of 10-100 nm. However, one such instrument, the electrical aerosol detector (EAD), has been found to exhibit a closer correlation with particle deposition (in terms of particle surface area) based on particle size. This result is confirmed by other instruments (i.e. a scanning mobility particle sizer and an ultrafine condensation particle counter) and a dosimetry model reflecting the tracheobronchial (TB) and alveolar (A) regions. Thus, an electrical aerosol detector or other diffusion charging instrument having a response near the Dp2 function can be used to take measurements over the 10-100 nm size range.
More demanding applications, for example matching mouth and nose breathing at different activity levels, and distinguishing among head region, A region and TB region depositions, require a closer correspondence to actual particle deposition within the lung and elsewhere in the respiratory system. For example, to assess certain health implications, it would be desirable to provide an instrument that more closely simulates the alveolar region as opposed to the tracheobronchial region. One reason, as noted above, is that cilia in the TB region tend to remove deposited particles, while the same particles would tend to remain in the alveolar region.